1. Field of the Invention
The invention is in the field of optical imaging, in particular, noninvasive near-field optical imaging.
2. Description of the Related Technology
Current state of the art optical near-field techniques are capable of obtaining only two-dimensional maps of optical intensity near the surface of a sample. Although in the case of homogeneous samples, such maps can be related to the sample's surface topography, their interpretation for manifestly inhomogeneous samples has proved to be problematic.
In principle, it is possible to extend these techniques in order to obtain three-dimensional studies of a sample's interior by sequentially etching thin layers of a sample starting from the top of the sample and performing near field scans of the sample's exposed surfaces. However, a drawback of this method is that the sample is destroyed and may be contaminated during the etching process.
Thus, near field microscopy is traditionally viewed as a technique for imaging surfaces. However, it has recently demonstrated the capacity to detect subsurface structure. Experiments in which a near-field probe is scanned over a three-dimensional volume outside the sample suggest that information on the three-dimensional structure of the sample is encoded in the measured data. That is, the measured intensity viewed as a function of height above the sample is seen to depend upon the depth of subsurface features. However, the intensity images obtained in this manner are not tomographic, nor are they quantitatively related to the optical properties of the medium.
The above noted difficulties have led to the use of inverse-scattering theory to clarify the precise manner in which three-dimensional sub-wavelength structure is encoded in the optical near field. Results in this direction have been reported for two-dimensional reconstruction of thin samples and also for three-dimensional inhomogeneous media. In either case, solution of the inverse scattering problem generally requires measurements of the optical phase in the form of a near-field hologram. Experimentally such a task is notoriously difficult and has not yet been realized.
A replacement of phase measurements by control of the phase of illuminating fields has also been proposed. In this approach, a collection of measurements of the power extinguished from incident evanescent waves with shifted phases is used to reconstruct the imaginary (absorptive) part of the dielectric susceptibility. Assuming that the experimental challenges of this approach could be resolved and such a technique could be realized, it is still impossible to recover the real part of sample's susceptibility using this technique.
Therefore there is a need in the field for a nondestructive method for optical imaging that does not expose a sample to undesired contaminants. It is also desirable that the method employs simple measurements of optical intensity in far-field and does not require interferometric techniques. The method may also be able to recover both real and imaginary parts of dielectric susceptibility.